The present invention relates to solenoids of the wet plunger type wherein the plunger is in a chamber that is open to the hydraulic fluid in the valve to which the solenoid is operatively connected.
It is conventional practice to actuate hydraulic valves by the use of dry solenoids wherein the push pin of the solenoid passes through a seal into the interior of the valve for moving one of the valve components, such as a spool. The electrical power requirements of these solenoids are relatively low, but the dry solenoids are unsatisfactory for use in many installations, because of problems that arise with respect to the seals. The seals have limited capacity, such as 1000 psi., for example, thereby limiting the capacity of the complete valve assembly to installations wherein the hydraulic pressures will not exceed the pressure capacity of the seals. Still further, the seals may develop leaks because of wear, shortening the life of the complete valve assembly.
To overcome these inadequacies of the dry solenoids, wet plunger solenoids have been developed wherein seals are omitted, and the plunger is movable in a chamber that is in direct communication with the hydraulic fluid within the valve. The chamber is formed by an armature tube and stop or pole pieces that are secured to the ends of the tube, and the high pressure hydraulic fluid can enter the chamber via the duct that extends through the one stop, nearest to the valve, and in which the armature push pin is positioned. The plunger can move in the chamber in response to the magnetic field produced when the solenoid is energized. Thus, the limitations placed on the valve by the seal are avoided, but additional problems are created.
Initially, the known wet plunger solenoids are faced with construction problems, because the plunger chamber must be sufficiently strong and leak-proof to withstand relatively high pressures, such as 3000 psi., for example. This condition makes it difficult to connect the armature tube to the stop pieces that form ends of the plunger chamber. Metal, such as stainless steel, is very desirable for manufacturing the tube, because of its non-magnetic properties and strength. The non-magnetic properties provide maximum possible hindrance to passage of the magnetic field axially through the midportion of the tube. Passage of a portion of the magnetic field through the tube would be an undesirable condition to exist, because the available field tending to move the plunger to actuate the valve would be lessened. However, in the prior art units the non-magnetic properties are harmful in other respects, because the tube, at its extremities passes through the magnetic field, providing an undesirable "air gap". To overcome this "air gap" problem, the prior art units require greater electric current to be passed through the coil, thereby causing overheating problems. Thus, the prior art units have been compelled to make the wall of the tube as thin as possible for the purpose of minimizing the thickness of the "air gap". Consequently, because of the thin nature of the tube, considerable difficulty exists in joining the tube to the stop pieces, this joining operation now normally being carried out by welding or brazing the parts together.
Efforts have been made to overcome the foregoing problems associated with wet plunger solenoids. One such effort is disclosed in U.S. Pat. No. 3,633,139, patented Jan. 4, 1972, in the name of Richard Thompson. The solution proposed in this patent is to heat treat the tube so as to establish a nonmagnetic band through the midportion of the tube and so as to establish magnetic properties in the end portions of the tube. This solution is explained in detail in the cited reference and will not be discussed further here, except to observe that the solution fails to solve fully the "air gap" problem, and further, it increases the difficulty of manufacturing the solenoid.
Thus, there still remains a need for an improved wet plunger solenoid that is capable of use with high pressure hydraulic valves and which will operate at relatively low electrical power to minimize power usage and avoid overheating problems.